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Thirteen oil workers were given prison sentences in connection with the violence

UN human rights chief Navi Pillay has called for an independent investigation into deadly riots last December in the western Kazakh oil town of Zhanaozen.

Ms Pillay, who is visiting Kazakhstan, said too many questions remained unanswered about the violence.

At least 15 people were killed and more than 100 injured when police opened fire on striking workers.

The unrest was Kazakhstan's worst since it gained independence from the Soviet Union in 1991.

"It is not clear who gave the orders allowing police to open fire, nor precisely why they did so", she said.

The shooting ended a seven-month-long strike by hundreds of oil workers demanding better pay and working conditions.

The events posed a serious challenge to the country's long-serving President, Nursultan Nazarbayev.

"I believe it is extremely damaging for Kazakhstan's reputation to have so much uncertainty hanging over such a serious episode resulting in substantial loss of life," Ms Pillay told a news conference in the capital, Astana.

"I have recommended to the government that the only way to credibly answer those questions once and for all, and draw a line under these tragic events, is to authorise an independent international investigation into the events themselves, their causes and their aftermath."

Spotlight

Last month, 34 out of 37 oil workers charged with organising and taking part in the mass unrest were convicted, and 13 of them, including a mother of four children, were jailed for up to seven years.

In a series of other trials, five policemen were given five- to seven-year prison terms for "abuse of power".

Ms Pillay, who is on her first visit to the Central Asia region, has spent two days in Kazakhstan meeting government officials, civil society groups and human rights activists.

Kazakhstan's Foreign Affairs Minister, Yerzhan Kazykhanov, has expressed his country's wish to join the UN Human Rights Council in 2013-2015.

"The country's candidacy for membership of the Human Rights Council will inevitably result in a spotlight being placed on its human rights record," said Ms Pillay.

Ms Pillay also criticised Kazakhstan's laws on assembly. She called for media and libel laws to be amended, saying that they currently force "a stifling form of self-censorship".

Oil-rich Kazakhstan is Central Asia's biggest economy, but the UN human rights chief says financial prosperity should not be sought at the expense of the full spectrum of civil rights.

Kyrgyzstan unrest

Before arriving in Kazakhstan, Ms Pillay visited neighbouring Kyrgyzstan, where she warned that discrimination against ethnic minorities could lead to a new wave of violence.

More than 400 people were killed in interethnic clashes between Uzbek and Kyrgyz communities in southern Kyrgyzstan in June 2010. Most of the victims were ethnic Uzbeks.

Ms Pillay said that the Uzbek minority continued to suffer from injustice.

"This imbalance is reflected in many key national and local institutions, including the police and the army," she said.

"It is perhaps most starkly illustrated by the June 2010 violence, during which around 75% of those killed were Uzbek, while some 77% of those arrested and charged with crimes relating to the violence were also Uzbek."